1. Field of the Invention
The present invention relates generally to the field of radiation sensitive resist films used in microcircuit fabrication and, more particularly, to the fabrication of positive electron beam sensitive resist films comprised of copolymers of methacrylic acid and t-butyl methacrylate having high sensitivity and submicron resolution.
2. Description of the Prior Art
The application of electron beam techniques to semiconductor fabrication has enabled great strides to be made in reducing the minimum line width and thus the size of an integrated circuit pattern which can successfully be manufactured. This has been accomplished both through progress in the technology involved including improvement in the precision of the electron beam system itself and in the progress which has been made in the technology concerned with the pattern defining medium or resist material.
In electron beam microfabrication, a substrate, which may be, for example, silicon dioxide, silicon, glass or chromium plated glass, is coated with a layer of polymer resist material. The resist is patterned by changing the solubility of the polymer with an electron beam. Subsequently, the resist film is "developed" by dissolving away the unwanted area of polymer utilizing a suitable solvent material and the resultant pattern is used as a mask for plating, chemical etching, ion etching or ion implantation. This electron beam lithography (EBL) is an integrated circuit production technique which utilizes a polymer resist to delineate circuit patterns for monolithic circuits.
When polymer resist materials of the required type are irradiated with an electron beam, the molecular structure is affected such that some of the polymer molecules are excited or ionized by the beam. This excitation causes some of the resist molecules to cross-link with other molecules in the polymer and others to degrade or undergo scission. The predominant manner in which such a polymeric material reacts to exposure to an electron beam has led electron resists to be classified into two main categories. A polymer which becomes predominately gelled or cross-linked, and thereby decreases its solubility after irradiation, is termed a negative resist. Conversely, if the scission process predominates and the solubility of the polymer increases after irradiation, it is called a positive resist. The resists of the present invention are positive resists.
A suitable electron resist material must have certain characteristic physical and chemical properties which allow it to fulfill the requirements for electron beam fabrication. The polymer material involved must be sensitive to an electron current of a fairly low value or the resist sensitivity itself will be the limiting factor on the writing speed and line width which can be achieved. The resist medium must be capable of achieving a high resolution or resist contrast compatible with that of writing and etching techniques utilizing the high resolution capability available with electron beam technology. In addition, the resist must have the ability to adhere satisfactorily to a variety of substrates used in different microfabrication applications. The medium also must be able to withstand normal acid, base, plasma and ion etching processes and should not be sensitive to small daily process variabilities.
Positive type electron resist films of comonomers of methylmethacrylate (MMA) with such monomers as acrylonitrile (AN), methacrylonitrile (MCN) and maleic anhydride are known in the prior art. Such combinations are shown in U.S. Pat. No. 3,914,462 to Morifshita et al. Similar copolymer materials are disclosed in a patent to Gipstein et al U.S. Pat. No. 4,011,351, which discloses a method of producing a positive resist image from copolymers of alkyl methacrylate units and polymerized units of certain other ethylenically unsaturated monomers. Those copolymers include alkyl methacrylate units wherein the alkyl group contains from one to four carbon atoms copolymerized with ethylenically unsaturated monomers which may contain any of numerous substituted groups. Certain other polymerized alkyl methacrylate copolymers including polymerized monoethylenically unsaturated acid units have also been proposed in Feder et al, U.S. Pat. No. 3,984,582.
The prior art such as the Feder et al patent, cited above, also discloses the use of a prebaking fabrication step in which the prepared polymeric resist, having been coated upon a substrate by spin coating or dipping and dried to remove volatile material, is prebaked. The prebaking temperature is above the glass transition temperature of the polymeric material, but below the decomposition temperature of the material to remove any remaining solvent. The prebaking step changes the solubility of the copolymer and improves adhesion to the substrate.
A prior application to Lai, J. H.; Douglas, R.; and Shepherd, L., Ser. No. 261,427, filed May 7, 1981, and assigned to the same assignee as the present invention, is directed to a method of making a different positive electron resist comprising a copolymer of methacrylic acid (MAA) and methacrylonitrile (MCN). Lai and Douglas are co-inventors in the present application.
The change in solubility of such copolymer resists has been found to be highly dependent on the prebaking temperature and time and is very hard to control. Thus, the time and temperature must be adapted to the particular polymer and desired result. Also, the temperature and time control in the prebake step must be extremely accurate in order to maintain reproducibility of resist sensitivity and resolution in the development of such resists.
Thus, it can be seen from the prior art that numerous polymers including copolymers of MAA and substituted MAA esters copolymerized with various ethylenically unsaturated monomers have been proposed. In addition, prebaking steps have been utilized in the preparation of resist films utilizing such copolymer combinations such as MMA/MAA with varying degrees of success.